[extropy-chat] Looking for examples of naturally evolved X-ray vision?

Amara Graps amara at amara.com
Wed Jan 18 17:05:59 UTC 2006

>My daughter asked me why the visible light spectrum IS the visible
>light spectrum. After all, animals hear at a wide range of frequencies
>that humans cannot, so why not have the same thing occurring in
>vision? Are there animals with X-ray vision?

>My first reaction was to say "no".

One can imagine with no stretch of current physics to have living
creatures on planets located around other stars, who evolve to have
Xray vision. This question is a beautiful question to answer with
elementary blackbody physics i.e. Wien's Law (*), spectroscopy (*), and
stellar evolution (*). The relationships I used below are simplified alot
(from an basic astronomy text), but they give a general idea for the
steps you can follow to quantify this problem.

Electromagnetic radiation with wavelengths between 400 and 700 nm is
called visible light because those are the waves to which human vision
is sensitive. This is the band of the electromagnetic spectrum where
the Sun give off the greatest amount of radiation. It's not coincidental
that human eyes evolved to see the kinds of waves that the Sun produces
most effectively. The more light there was by which to see, the more
efficiently humans could evade predators and make babies before they
got stepped on or eaten. Visible light penetrates the Earth's atmosphere
effectively, except when it is temporarily blocked by passing clouds.

 From Wien's Law, the wavelength at which a blackbody emits its maximum
energy can be calculated according to :

max wavelength (nm) = 3x10^6 / T (K).

The temperature (surface) of the Sun is 5800 K, so then the wavelength
at which the maximum energy is emitted is: 520 nm, which is in the
middle of the visible light range.  This explains much of the wavelength
of human vision and probably the range of other Earth animals vision,
as well.

For Xray vision, the middle of the X ray wavelength range is 1 angstrom,
10^{-10) m = 0.1nm

To evolve 'naturally', one needs to have a nearby star with the following
temperature: T(K) = 3x10^6 / 0.1nm = 30,000 K.

You can identify such a star from the Hertzsprung Russell diagram (*).
It is an O or (or hot B) spectral class star, which has a luminosity
of ~ 1000 * solar luminosities.

 From the main sequence luminosity, you can approximate the star's

L ~ M^{3.5} ==> M ~ 7 solar masses

It will have a lifetime on the main sequence (for living creatures
to evolve):

t ~ 1/ M(M_solar)^{2.5) = 0.01 solar lifetimes

Since the solar lifetime is about 10 billion years = 10^{10} years,
then the lifetime of this star is about:

10^{-2} * 10^{10} = 10^8 years (=100 million years)

When this star evolves off of the main sequence, it will be a giant
or supergiant star.


(*) http://en.wikipedia.org/wiki/Optical_spectrum
(*) http://en.wikipedia.org/wiki/Hertzsprung-Russell_diagram
(*) http://en.wikipedia.org/wiki/Wien's_displacement_law
(*) http://en.wikipedia.org/wiki/Stellar_evolution

Seeing the Whole Symphony  (a beautiful lesson about Spectroscopy)


Amara Graps, PhD
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF),
Adjunct Assistant Professor Astronomy, AUR,
Roma, ITALIA     Amara.Graps at ifsi.rm.cnr.it

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